1. Field of the Invention
The present invention relates to a mechanism for fixing a photodetector for detecting reflected beams from a recording medium on a base plate.
2. Description of Related Art
FIG. 1 is a perspective view showing disassembled parts of the conventional mechanism for fixing a photodetector disclosed in, for example, the Japanese Patent Laid-Open Gazette No. 230632 (1986).
In the drawing, reference numeral 1 designates a mounting member for fixing a photodetector 1A to a base plate 2. At both the lateral ends of the base plate 2 are provided threaded bores 4a, 4b where two screws 5a, 5b for fixing the mounting member 1 to the base plate 2 are to be screwed. At the central portion of the base plate 2 is provided a guiding hole 7 for guiding the reflected light flux 6 to the photodetector 1A from a recording medium (not shown). The mounting member 1 has through bores 3a, 3b through which the screws 5a, 5b perforate, respectively, being provided at the positions corresponding to the respective threaded bores 4a and 4b.
Next, how to fix the photodetector 1A to the base plate 2 by the mounting member 1.
The photodetector 1A is temporarily disposed by being mounted on the mounting member 1. The reflected light flux 6 reflected from a recording medium passes through the guiding hole 7 at the base plate 2 to reach the photodetector 1A. The reflected light flux 6 from the recording medium includes the servo information and the reproduced information. In order to obtain with accuracy the servo information or reproduced information, the position of the photodetector 1A is finely adjusted. The screws 5a, 5b are screwed up in the threaded bores 4a, 4b at the base plate 2 through the through bores 3a, 3b at the mounting member 1, respectively thereby fixing the photodetector 1A to the base plate 2.
When the friction coefficient between the base plate 2 and the mounting member 1 is represented by .mu.1 and that between the mounting member 1 and the heads of the screws 5a, 5b by .mu.2, and when .mu.1 and .mu.2 satisfy the following inequality: EQU .mu.1&gt;.mu.2 (1)
the heads of the screws 5a and 5b tighten the mounting member 1 while sliding thereon. On the other hand, since the mounting member 1 does not slide against the base plate 2, the photodetector 1A is not shifted.
However, the friction coefficients .mu.1 and .mu.2 largely depend on the surface condition, whereby they do not inevitably satisfy the inequality (1).
In a case where engineering plastics, such as PPS (Poly Phenylene Sulfide), is used as material of mounting member 1, the friction coefficient of PPS is generally smaller to an extent of being usable as a base plate substitute for fluoroplastics, whereby a slight variation in the surface condition results in the inequality (1) not being satisfied.
The conventional photodetector fixing mechanism constructed as above-mentioned has the problem that, when the friction coefficient .mu.1 is smaller than .mu.2, the mounting member 1 slides against the base plate 2 during fixing the mounting member 1, whereby the positioned photodetector 1A may be shifted due to turns of the screws 5a, 5b.